Vinyl is commonly used in seating systems and other articles. A more durable alternative to vinyl is polyurethane but generally speaking polyurethane is a porous material and does not have sufficient stain, abrasion and chemical resistance. Furthermore, vinyl is commonly used in the healthcare environment such as hospitals or the like, as material for seating and beds. However, vinyl is easily punctured or torn and over time the exterior surface of vinyl beds and chairs break down, so as to provide areas where bacteria, viruses, bed bugs or the like can breed.
The present invention relates to a chemical resistant grafted coating applied to polyurethane for use in seating, beds and other articles. It also relates to a method of grafting a protective coating on polyurethane parts for stain, abrasion and chemical resistance, water resistance and other adverse effects of the environment.
Even when the polyurethane is coated with commercial coatings, it does not have the desired properties. These coatings adhere to the substrate through only physical bonds, that can be readily dislodged from the polyurethane substrate over a short period of time as moisture, oxygen and other corrosive gases permeate beneath the physically bonded commercial coating on the polyurethane substrate. Thus, there is a need for a chemically bonded coating for polyurethane against abrasion and staining, healthcare cleaning and disinfecting agents and to reduce porosity.
General
Grafting by definition means to transplant or to be transplanted from one body to another in such a way that the new growth can continue. Similarly, chemical grafting involves the transplantation of monomers to various substrates, to improve their positive properties without any basic change to the substrate itself.
The invention as described herein uses monomers/prepolymers as grafting components and graft initiators to start and continue the grafting process.
Originally, grafting has been done by using various types of electromechanical and electronic devices which performed very well, but as the pace of technological advancement quickened, such equipment became either too expensive or inadequate. Prior art devices that have been and still are used are: gamma radiation, X-ray, Corona discharge, sputtering and other similar methods.
Chemical grafting on the other hand generally relates to chemical bonding and may be divided into three basic areas depending on their functional capability. They are:
1. Coatings
As the term implies, surface characteristics of a substrate are changed by applying a coating. Coatings may be used to improve the appearance of the substrate, for corrosion protection, for electrical insulation, or adhering two materials together and the like. In each case, conventional coatings adhere to substrates by simple physical forces which can be easily broken, and consequently peeling or delamination may occur.
Such is not the case in chemical grafting
Since the attachment of coatings is accomplished by forming a chemical, covalent bond, much thinner coatings can provide extended life and superior adhesion. Successful grafting of a multitude of monomers and substrates such as metals, plastics, rubber, cellulosic materials, liquids and nature formed, living organisms can be accomplished. The chemical reaction that takes place on the surface provides a monomolecular layer of chemically bonded coating, which can be increased in thickness to the desired amount. Results have shown that coatings of ½ mil thickness can provide up to 2000 hours of salt spray protection.
2. In Depth Grafting
In many instances the fine layer of grafted material which can be easily deposited on the surface is not sufficient to alter and improve on the existing substrate. Sometimes it is necessary for the monomers to penetrate into a certain depth or even throughout the matrix of the substrate to perform correctly. For example, to achieve total wettability of plastics such as porous Teflon, nylon and many others, one must be able to chemically graft water absorbing chemicals in depth. Similarly, to obtain permanent flame retardancy, one must totally penetrate a textile or a bundle of yarn filaments or even saturated wood.
3. Laminating
In a multitude of applications it is necessary to combine more than one substrate together in a form of a laminate. Sometimes several layers of various materials are laminated (sandwiched) together to eventually have the specified characteristics. An off-the-shelf adhesive (glue) that is normally used for such applications will typically fail when temperature changes occur. The adhesive will fail in shear, when various layers comprising the laminate have different coefficient of thermal expansion.
To avoid such failures a group of graft initiators have been developed which can be instrumental in grafting difunctional monomers in such a way that one of the carbon-carbon chains will attach itself to one of the substrates at temperature T1 and the other end will follow suit at some temperature T2, higher than T1. It is known, that carbon-carbon chains are in the form of a helix, which can extend and contract depending on temperature.
By utilizing this technology even substrates such as glass ribbon and polypropylene have been sandwiched and subjected to thermal shock without showing ill effects. In total, molecular grafting is a tool for the industry which permits alterations of the properties of substrates inexpensively, with very small dimensionally stable coating which outlasts all conventional and expensive ones.
Chemical Grafting
The idea that a second polymeric species can be attached by a covalent linkage to an existing polymeric material was first suggested in the late 1930's. A substance of this type was first produced in the laboratory in the early 1940's. Since that time, sufficient data has been accumulated about such processes, so that they have gained an important position for a variety of industrial applications. This method, where a “foreign” material becomes attached to another material by means of a chemical bond is referred to as “chemical grafting”. One example is the production of acrylonitrile-butadiene-styrene (ABS) resin obtained by the direct attaching of styrene-acrylonitrile on to a polybutadiene backbone. This often is achieved by the polymerization of styrene and acrylonitrile in the presence of butadiene and is an example of chemical bonding technology.
Chemical grafting might be visualized as the growth of “whiskers” onto a material. These whiskers are joined to the substrate (basic material) by means of a chemical bond. This is in direct contradiction to ordinary coatings where the bond between the substrate and the coatings is only physical in nature. By chemical grafting a much higher degree of permanency is achievable.
Chemical grafting involves the activation of the substrate. Once the substrate has been activated, chains of monomers linked by carbon bonds grow on the substrate.
There are a number of patents that relate to chemical methods that allows for the activation and attachment of a wide variety of monomers to fabrics (for example, cotton, rayon, nylon, polyester, fiberglass, acrylics, polypropylene and the like), wood, paper, cellulose, metals (steel, copper, brass, lead aluminum, silver, zinc, etc), glass, plastics (polyethylene, polypropylene, Teflon, polyvinyl chloride, polycarbonate, polyethylene terephthalate, etc), Kevlar, biopolymers, hair (human and animal), human tissue (skin), liquid resins, liquid polymers, and the like.
For example U.S. Pat. No. 5,552,472 teaches a fabric containing graft polymer thereon, while U.S. Pat. No. 6,638,319 relates to a polymer for printed cotton. Also U.S. Pat. No. 6,482,529 relates to polymeric coating compositions, polymer coated substrates, and methods of making and using the same.
Most often, no new equipment or processing steps are required to carry out the process. This method differs from many other methods of chemical grafting which require radiation, high or low pH, and the like.
Finally, the question of pollution is taken into account. Where possible, and this is generally the case, the reactions for chemical grafting make use of emulsions or aqueous solutions. Towards this end, methods have been developed to solubilize the necessary organic materials in water. Also, by the time the process is complete, the organic materials are essentially exhausted.